Interesting convo. Cloud seeding definitely occurs, but that involves smaller aircraft releasing condensate such as silver iodate into a developing cloud base. Boeings releasing thin streaks of condensate at high altitude for hundreds of miles, would be the most expensive, least effective way of modifying climate. Besides, where would a commercial aircraft like a Boeing store the hundreds of liters (and kg) of the required chemicals? Just makes no sense.

Also, bear in mind that there were fewer planes when we were growing up, and I'm not sure they flew at the same attitudes as now. When someone shows conclusive evidence of condensate chemicals and a release mechanism on a COMMERCIAL JET PLANE such as a Boeing, I'll gladly reassess my skepticism.